1. Field of the Invention
The present invention relates to processes for preparing dry crystals of .alpha.-L-aspartyl-L-phenylalanine methyl ester (aspartame).
2. Discussion of the Background
Aspartame is a refreshing sweetener which has the salient advantage of having a very low level of the bitter taste or aftertaste generally found in highly sweet sweeteners. It has thus found wide use as a low calorie sweetener.
In terms of its physical properties however, aspartame suffers from poor dispersibility and solubility in water. To obtain aspartame having excellent solubility, investigators have tried to form it into granules, effervescence foam tablets and the like by combining it with excipients or disintegrators.
However, the presence of these excipients, etc., is often troublesome depending upon the intended use of the product. A high purity aspartame product with good solubility characteristics is thus in strong demand.
Attempts have been made towards improving the solubility of aspartame while maintaining its purity. These attempts include a method which comprises spray-drying aspartame in its slurry state (Japanese Patent Publication No. 58-20588), a method which comprises adding water to aspartame in a specific water content and granulating the aspartame (Japanese Patent Application Laid-Open No. 59-95862), etc.
With respect to the solubility of aspartame crystals per se, the I.sub.B type crystals disclosed in Japanese Patent Application Laid-Open No. 59-172444 have good solubility as dry crystals. (I.sub.B type aspartame crystals are described in detail in EP 0 119 837 and U.S. Pat. No. 4,579,747.) However there is still a need for a method to obtain pure I.sub.B type crystals on an industrial scale. To date no such process for obtaining high purity I.sub.B type crystals is available.